Variator-assisted CVTs are known and have been primarily devised in order to replace a conventional CVT in applications where a wide spread of gear ratios are desired. To do so with a conventional CVT would mean providing a CVT which is large and heavy, which is undesirable in a vehicle. Variator-assisted CVTs work by taking a torque received at the transmission input shaft from an engine and splitting that torque into two paths: one which is directed to the transmission output through a summing transmission alone, and the other which is directed through the variator and the summing transmission.
One limitation of some variator-assisted transmission (VAT) configurations is that it is not possible to achieve a zero ground speed (i.e. where the vehicle is not moving) using the variator without slipping or disengaging a clutch. Thus, a vehicle employing such a transmission will always “creep” if a clutch of the transmission is engaged. Reducing the input engine speed to the transmission reduces the amount of creep but cannot eradicate it without shutting down the engine.
One way of overcoming this limitation is to connect the variator output side directly to the output shaft by way of a variator clutch. In such an arrangement the variator would be connected to the input side of the summing transmission as normal, but could be selectively connected directly to the output shaft when the clutch between the summing transmission output and the output shaft is disengaged. The VAT can then not only achieve zero speed with a clutch engaged but can also provide very low output speeds for a crawling/inching mode, and launch the vehicle from zero output speed into the standard transmission modes by varying the output of the variator. One disadvantage of this solution is that the additional parts required to connect the variator in this way increase the overall cost and complexity of the transmission.
VAT configurations exist which can launch from zero speed, but each has its own disadvantages. One way is to manipulate the gear ratios of the VAT but this reduces the maximum travel speeds in forward and reverse modes. An alternative is to utilize a variator with higher speed capacity, but such a variator would be more expensive.
It is an aim of the present invention to obviate or mitigate one or more of these disadvantages.